1. Field of the Invention
The present invention relates to a digital frequency divider, and more particularly to a programmable digital frequency divider employing a low frequency crystal and an oscillating circuit for generating an output clock signal with a precise divided frequency.
2. Brief Description of the Prior Art
It is an inevitable trend to design miniature and low cost integrated circuits in the future. It is an important point not only to improve the internal circuit arrangement of the integrated circuit but also to enhance the performance of the integrated circuit and reduce the manufacture cost.
It is known that a piezoelectric crystal is used to supply a source of basic clock signals to provide local timing control for a crystal based oscillator. The oscillator frequency is essentially determined by the crystal.
FIG. 1 illustrates a functional block diagram of a conventional digital frequency divider which mainly includes a crystal 10, a high frequency oscillating circuit 11, and a plural stages of frequency dividing units 12a, 12b, 12c, etc. The frequency dividing units 12a, 12b, and 12c are capable of generating various output clock signals with different frequencies on respective clock output lines 13a, 13b, 13c, etc. It is noted that this type of conventional digital frequency divider has the following limitations:
1. A high frequency crystal must be used in order to obtain a variety of output clock signals with different frequencies, which means that the component cost is relatively high. PA1 2. The conventional divider is limited to provide an output signal with a frequency equal to the frequency of the input clock signal divided by an integer. In case that the divider can not generate an output signal with an exactly required output frequency, another crystal element with higher frequency must be used. PA1 3. It is required to re-design the logic circuit of the divider once the system frequency of the divider is changed, which will waste much time in circuit design. In addition, the specifications of the major electronic components of the divider must be changed or modified.
It is obvious that the drawbacks of the conventional frequency divider described above are extremely disadvantageous to the integrated circuit product both in scale and manufacturing cost.